Provided is a waveguide converter, an antenna apparatus and a radar apparatus. The waveguide is provided with a simple structure for guiding a microwave from a waveguide to other waveguides, which is easy t o manufacture. The waveguide converter (30) includes elongate inner side conductor (31) having specific thickness, a first portion (311), a second portion (312), a third portion (313) and a support portion (314), wherein the first portion (311) is at one end side of the elongate direction, has a first width, and is exposed for introducing waveguide portion (10); the second portion (312) is at the other end side of the elongate direction, has a second width narrower than the first width, and is exposed from a waveguide portion (20) for feed; the third portion protrudes from a proper position of the first portion (311); and the support portion (314) extends a prescribed length in a perpendicular direction at least from one of the first portion (311) and the third portion (313).